Jim, the Medical Miracle

Balto successfully delivered the diphtheria antitoxin, but a horse named Jim pioneered it. Jim had a modest job pulling a milk wagon when, at the turn of the 20th Century, he contracted diphtheria. But he didn't die, and doctors discovered that his body had developed antibodies to fight the disease. They used Jim's awesome immune system to make a serum for humans, with wide success! That is... until Jim contracted tetanus (unlucky horse!). It was discovered that much of the recent serum extracted from Jim's super-blood had been contaminated with tetanus, leading to the deaths of some of the inoculated patients. This screw-up led to the Biologics Control Act of 1902, which stipulates that before you chuck some junk into a person's body, you should probably test it for harmful diseases.